1. Field of the Invention
The subject of the invention is a process for the electrodeposition of a chromium coating containing solid inclusions; the aim of the invention is also to provide the plating solution employed in this process.
Chromium electroplating is used because of the improvement in the physical surface properties which it provides when it is used. In particular, these improvements are significant, without this list being limiting, in the fields of:
wear and abrasion resistance, PA1 corrosion resistance, PA1 coefficient of friction PA1 hardness. PA1 very high current density, PA1 use of addition cations, PA1 periodic reversal and codeposition during the anodic phase, as described in Patents EP 0 217 126 and U.S. 4 846 940. PA1 the results are not reproducible in the majority of cases, this being so even by using the same experimental conditions, PA1 the inclusions are not distributed homogeneously in the plating; they are often concentrated in the plating cracks caused by the relieving of internal stresses and their opening during the anodic phase (in processes using periodic current reversal). This heterogeneous distribution of the particles can introduce malfunctions prejudicial to the characteristics which it is sought to improve, PA1 the concentration of included particles remains limited to a few percent, PA1 these methods require a lengthy and expensive preparation of the particles.
It is for these various reasons that chromium electroplating is used when components are in a frictional environment or exposed to certain chemical attacks or when it is necessary to maintain a shiny and smooth surface finish.
It has been known for a long time that it is possible to modify the characteristics of certain metal platings (copper and nickel deposited electrolytically and chemically, for example) by codepositing particles of variable size and nature, and to do so according to the characteristics which it is sought to improve.
2. Description of the Prior Art
Many authors have described the codeposition in nickel and copper platings of particles of oxides, carbides, nitrides, metallic borides and organic compounds of a size varying from a few angstroms to a few tens of microns. They have demonstrated the overall advantage of this, in particular in the fields of wear resistance, improvement to the coefficient of friction, and corrosion resistance.
In this regard, an appreciation of the methods used and of the results obtained is given in the literature. For example, U.S. Pat. No. 3,844,910 describes a process for obtaining nickel and cobalt coatings containing, as solid inclusions, silicon carbide in the presence of aminosilicates.
The application of this technique to chromium plating therefore seem to be entirely indicated since it should provide the same type of improvement to these layers, the physical properties of which, before the addition of particles, are in many fields superior to those obtained with the aforementioned metals containing inclusions.
In fact, it is also known, although more difficult to implement, to be able to codeposit as inclusions, in chromium plating solutions, various types of particles of variable size. Some authors describe production methods and the physical characteristics of the platings obtained.
By way of example, U.S. Pat. No. 1,098,066 describes a method of obtaining a chromium coating containing, as solid inclusions, aluminum oxide, titanium oxide or a mixture of Ca.sub.2 Al.sub.2 Si.sub.3 O.sub.8 and of Al.sub.2 O.sub.3. 2SiO.sub.2. This coating is produced by electrodeposition from a plating solution comprising chromium in the oxidation state VI. In the examples given in this patent, the chromium in the oxidation state VI comes from chromic anhydride.
Likewise, GB Patent 1,220,331 describes a process for obtaining a coating consisting of a chromium matrix containing, as solid inclusions, ceramic particles, metal particles or a mixture of metal particles and ceramic particles. Here too, this coating is produced by electrodeposition from a solution comprising chromium in the oxidation state VI. In the example given in this patent, the chromium in the oxidation state VI comes from chromic acid.
However, all such platings of chromium layers with inclusions of particles are produced using solutions of hexavalent chromium (chromium in the oxidation state VI), in particular aqueous solutions of chromic anhydride (CrO.sub.3), these solutions varying in the nature of the catalysts used. The differences in the methods, apart from the nature of the catalysts, are essentially due to the conditions of use:
Drawbacks arise insofar as:
However, despite all these imperfections in the platings obtained, despite the fact that the hexavalent chromium solution is very toxic (in terms of disposal, the toxicity is classified as equivalent to cyanides) and despite the fact that this codeposition is difficult, the improvements in the physical properties of the chromium depositions with inclusions obtained are such that they justify the industrial use thereof.
There is therefore a major industrial advantage in being able to codeposit, homogeneously, reproducibly and simply, particles of different size and nature depending on the properties which it is sought to improve.
This advantage would be even greater if it were possible to avoid the use of solutions of chromic acid, which is a strong and highly oxidizing acid (and therefore dangerous to use) and above all, is carcinogenic and highly toxic.
The development of a process allowing the use of a trivalent chromium solution as electroplating solution for the codeposition of particles in chromium platings would therefore seem to be of great utility, insofar as trivalent chromium solutions are reputed to be less toxic (the toxicity of trivalent chromium solutions is of the same order of magnitude as that of the usual metals) and insofar as, in addition, for the same current efficiency (Faraday yield), the rate of deposition of chromium from a trivalent chromium solution is twice as rapid as that obtained using a hexavalent chromium solution. In other words, for the same current efficiency (for example 25%), for a current density of 50 A/dm.sup.2, the rate will be of the order of 1 .mu.m/min for a plating obtained using a hexavalent chromium solution, as it will be twice that (2 .mu.m/min) for a trivalent chromium solution.
Many patents and publications describe methods for allowing layers of chromium to be obtained by electroplating from trivalent chromium solutions, the solutions in question not comprising particles. To our knowledge, the only existing industrial applications are in the field of decorative chromium. Very few publications or patents take account of the possibility of obtaining thick, hard and dense layers; generally the layers obtained are friable, adhere poorly or have a hardness of the order of 700 Hv/100 g and, in any case, markedly inferior to the hardness (of the order of 1000 Hv/100 g) of the chromium obtained from hexavalent chromium plating solutions.
It will be recalled that Hv is the "Vickers hardness", a unit of force by which the hardness of the substrate is expressed.
Among these few written documents, Patents EP 0,099,793 and U.S. Pat. No. 4,612,091 describe a method allowing thick and hard layers of chromium to be deposited by using a trivalent chromium solution obtained by reduction of hexavalent chromium by means of a reducing agent chosen, inter alia, from alcohols, hydrogen peroxide, hyposulfites and sulfur dioxide, these solutions containing no complexing agents. In these patents, hydrohalic aqueous solutions are used as reaction medium. Moreover, it has also been shown that other media, (instead of hydrohalic compounds) could be used for the reduction, for example: sulfates, nitrates, fluoborates and organic acids, without this list being limiting.
To the knowledge of the Applicant Company, there are no publications or patents describing a process and/or a plating solution enabling chromium and particles of any kind whatsoever to be codeposited by means of an aqueous trivalent chromium solution obtained by reduction of chromic acid.
The Applicant Company has carried out various types of tests in order to examine the possibilities of codeposition of particles in a chromium plating from a trivalent chromium solution, the chromium being obtained by reduction of chromic acid, and, to do this, it has attempted to codeposit particles of different kinds and sizes in trivalent chromium solutions obtained by reduction by means of reducing agents such as alcohols, hydrogen peroxide, hyposulfites and sulfur dioxide in hydrohalic acid, sulfuric acid or organic acid (formic or acetic acid) medium, without this list being limiting.
Since in general trivalent chromium is more difficult to deposit than hexavalent chromium and since it is difficult to obtain a composite coating of sufficient quality from a solution containing hexavalent chromium and uncharged particles, the electrolysis of these aqueous trivalent chromium solutions obtained by reduction of chromic acid, to which the particles of kinds and of sizes to be tested had been added, has surprisingly enabled the Applicant Company to obtain dense, adherent, semi-bright, smooth and hard (hardness greater than 1000 Hv/100 g) and non friable layers which appear to be metallic chromium layers containing particles as inclusions when the plating is carried out on a substrate, this substrate being placed as the cathode.